Eddy current testing is a non-destructive test technique based on inducing electrical currents in the material being inspected and observing the interaction between these currents and the material. Eddy currents are generated by electromagnetic coils in the test probe, and monitored simultaneously by measuring probe electrical impedance. Since it is an electromagnetic induction process, direct electrical contact with the sample is not required; however, the sample material must be electrically conductive.
When inspecting for defects, it is essential that flow of eddy currents be as perpendicular as possible to defects to obtain maximum response. If eddy currents flow parallel to a defect, there will be little distortion of the eddy currents and hence little change in probe impedance.
Various eddy current probes have been proposed for inspecting cylindrical or tubular components as seen in U.S. Pat. Nos. 3,952,315 Apr. 20, 1976 (Cecco), 4,079,312 Mar. 14, 1978 (Osborne et al) and 4,083,002 Apr. 4, 1978 (Allport).
A conventional internal circumferential probe induces a flow of eddy currents parallel to the coil windings and therefore circumferential in direction. As mentioned above, coil impedance must change to sense a defect. This will occur if eddy current flow path is disturbed. Circumferential defects parallel to this current, which present no significant area perpendicular to this path, will therefore not be sensed. Multiple coils in excitation coil assembly and in receiver coil assembly are also described in U.S. Pat. Nos. 3,241,058 Mar. 15, 1966 (Quittner) and 3,271,662 Sept. 6, 1966 (Quittner). The above two patents to Quittner teach sheet metal inspection using an odd number of coils with their axes perpendicular to test sample for excitation and an even number of coils for receiving. The excitation coils are electromagnetically polarized alternately but the receiver coils are polarized in same directions and therefore no circumferential or line compensation is possible (a desired feature in the Quittner patent). They also produce complicated output signals to analyze and are not readily applicable for cylindrical testing.
U.S. Pat. No. 3,444,459, May 13, 1969 (Prindle et al) describes helical sensing coils slightly skewed relative to the axis of the tube. The sensing coils are alternately polarized but must be in an elongated shape for 100% circumferential coverage and manageable axial probe length. The excitation coil is at least three times larger than the sensing coil coil assembly. The probe has no sensitivity to circumferential cracks.
The present invention makes use of an excitation coil for each set of multiple receiver coils operating in the transmit-receive mode for detecting defects in a cylindrical tube.